Edoc - A Technical Basis for Guidance on Lightning Protection for NPP

7/28/2019 Edoc - A Technical Basis for Guidance on Lightning Protection for NPP

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Fourth American Nuclear Society International Topical Meeting on Nuclear Plant Instrumentation, Controlsand Human-Machine Interface Technologies (NPIC&HMIT 2004), Columbus, Ohio, September, 2004

A TECHNICAL BASIS FOR GUIDANCE ON LIGHTNING PROTECTION

FOR NUCLEAR POWER PLANTS

R. A. Kisner, J. B. Wilgen, P. D. Ewing, K. Korsah, and M. R. Moore

Oak Ridge National LaboratoryP.O. Box 2008, MS 6006

Oak Ridge, TN 37831E-mail: [email protected], [email protected], [email protected],

[email protected], and [email protected]

Keywords: lightning, nuclear power, surge protection

ABSTRACT

This paper describes the underlying basis for guidance on the protection of

nuclear power systems from direct lightning strikes to facilities and resulting transienteffects. Oak Ridge National Laboratory has been engaged by the U.S. Nuclear

Regulatory Commission Office of Nuclear Regulatory Research to develop a technicalbasis for guidance to address design and implementation practices for lightning

protection systems in nuclear power plants (NPPs). Lightning protection is becoming

increasingly important given the use of digital and low-voltage-analog systems in NPPs.Although these modern systems have advantages and useful features, they have the

potential to be more vulnerable than older, analog systems to the power surges andelectromagnetic interference that result when lightning strikes facilities or power lines.

1. INTRODUCTION

The U.S. Nuclear Regulatory Commission (NRC) Office of Nuclear RegulatoryResearch (RES) has engaged Oak Ridge National Laboratory (ORNL) to develop a

technical basis for guidance that addresses design and implementation practices for

lightning protection systems in nuclear power plants (NPPs). Lightning protection inNPPs is becoming increasingly important with the widespread use of digital and low-

voltage analog electronic systems. For a variety of reasons, these systems have positivebenefits in nuclear power systems; however, because of lower internal operating voltages

and high internal operating frequencies, they are potentially more vulnerable to lightning-

induced transients than are older, analog systems. This paper discusses important issuesregarding the protection of nuclear power instrumentation and control (I&C) systems

from direct lightning strikes and resulting secondary effects.The scope of the ORNL technical basis includes protection of the power plant and

relevant ancillary facilities, with a boundary beginning at a buildings service entrance,

including (1) the in-plant electrical distribution system, safety-related I&C systems, andcommunications within the power plant, as well as (2) other important equipment in

remote ancillary facilities that could impact safety. The scope excludes protection ofpower lines and the plant switchyard. It does not cover testing and design practices

intended to protect safety-related I&C systems against the secondary effects of lightning

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discharges [i.e., low-level power surges and electromagnetic interference (EMI)]. These

practices are covered in Regulatory Guide 1.180 (NRC 2000a).Six Institute of Electrical and Electronics Engineers (IEEE) industry standards are

recommended for endorsement to address issues associated with NPP lightningprotection. In addition, reference data from several other lightning-related standards are

included in the recommendation.

1.1 A Primer on Lightning

Weather experts report that lightning strikes the earth 100 times each second

around the world and that 16 million thunderstorms occur worldwide each year (When

Lightening Strikes, 2000). The

regions most prone to this violentweather are those where very moistand unstable air masses move through

year-round (e.g., regions in close

proximity to the Gulf of Mexico andthe Atlantic Ocean) (Schill, 1996).Some additional facts about lightning

are shown in Fig. 1 (LPI, 1999; Uman,1984).

Some U.S. Department of

Energy nuclear facilities in theSoutheast have been known to

experience more than 75 lightning-related occurrences over a 5-year period, while othersin the West escaped with just 9 over the same period (Schill, 1996). In the nuclear power

industry, a lightning strike at the Diablo Canyon Nuclear Power Plant on September 22,

1999, caused a reactor trip (Tenth Annual Report on the Safety of Diablo Canyon NuclearPower Plant Operations, July 1, 1999June 30, 2000). In addition, a reactor at the Rivne

Nuclear Power Plant in Ukraine automatically shut down on August 18, 2000, after it wasstruck by lightning (Nuclear Reactor Shuts Down after Lightning Strike, 2000).

Lightning is a serious threat, and protection is essential to avoid malfunctions and upsets

that, in turn, lead to reactor trips and unsafe conditions. It is generally believed by somethat increases in global warming will lead to more frequent and severe storm activity.

Fig. 1 Lightning facts.

1.2 Lightning Effects and Propagation

Procedures and requirements to prevent lightning-induced transients frompropagating to and interfering with safety-related I&C systems contribute to the

assurance that structures, systems, and components important to safety are designed toaccommodate the effects of environmental conditions (i.e., remain functional under all

postulated service conditions). The most important deleterious effects of lightning-

induced transients are that systems related to safety are either rendered inoperable or aretriggered to spurious operation, and that fires are started, which damage crucial

equipment or impede safety-related functions.

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The scope of our investigation includes medium- and distribution-level voltages

within the nuclear plant that power safety-related I&C systems and components. Theboundary as far as lightning guidance is considered is at the interface between the

substation and the building. For example, general protection of the substation againstdamage from lightning would not be a part of the investigation.

Electrical distribution from off-site power through control and safety systemsforms a series path through the plant, as shown in Fig. 2. Equipment housed in the plant

building structures is protected from direct lightning strikes; however, propagation oflightning-induced transients may follow a variety of paths from the switchyard tosensitive electronic components owing to its high-voltage, high-current, and high-

frequency characteristics. IEEE standards related to lightning protection have overlapping

applicability to NPP systems; they are shown in Fig. 2.

Fig. 2 Applicable IEEE standards associated with plant components.

A lightning strike generates transients that propagate through plant electrical

distribution and are coupled into equipment by a variety of mechanisms, such as directcoupling through conductors, magnetic induction to nearby conductors, capacitive

coupling, direct arc-over to lower-potential points, and ground potential rise (GPR).Equipment that can be affected ranges from power-level devices (e.g., motors, circuit

breakers, and transformers) to electronic I&C systems. I&C systems are particularly

vulnerable to transients, which can enter through power supply connections and sensorsignal paths. The unique feature of a lightning transient is that it does not interact with

facilities and components in a consistent manner.The possible consequences of a lightning strike are summarized and generalized

in Fig. 3. Lightning induces transient currents and voltages that produce a direct primary

effect on plant systems and components. As a result, system and component responsesinclude tripping of protective relays, spurious response, component failure, and fire.

Ultimately, a significant action results, such as reactor trip, spurious safety actuation,blockage of safety actuation, or loss of fire protection. No experiential data from plant

records have shown an example of blocked safety system actuation, although some

lightning events have come close to such a response.

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2. LICENSEE EVENT

REPORT DATA

A previous examination ofthe NRC licensee event report

(LER) database included 174 eventswith lightning-related incidents at

NPPs from the period 1980 to 1991(Rourk 1992, 1994). The conclusionof that examination was that the

most significant impact on plant

operation that may be lightning-

caused is from the effects of localstrikes, such as GPR. High-frequency voltage transients created

on the transmission system by

lightning do not cause significantequipment misoperation or damagebecause they are greatly attenuated by redundant levels of protection before reaching the

low-voltage service level. Based on the analysis of LER data, Rourk (1992 and 1994)concluded that local strikes do not appear to create a significant risk to plant safety.

Fig. 3 Effects of lightning transient on plantsystems.

A second LER search was made by the ORNL team to determine the effects of

lightning over the next 10 years, 1990 to 2000. Table 1 summarizes the categories of suchlightning-induced events. Nineteen plants reported a total of 30 events that listed

lightning as the causal agent. Eleven events involved reactor trips, 9 involved a loss ofpower or other action that caused the backup diesel generator to start, 2 each involved

ventilation or containment isolation, and 6 others were miscellaneous events.

It appears significant that while less than 20 percent of the NPPs reportedlightning-induced LERs, several of those plants recorded two or three during the 1990s.

This issue should be pursued to determine whether these plants have specialsusceptibilities or whether they fall within the expected norms, given their local weather

conditions. Similar discrepancies between lightning events at nuclear power plants were

observed in a study of four plants by Entor Corporation (Pielage, 1981). The differenceswere attributed to the degree of adherence to lightning protection practices prescribed in

National Fire Protection Association (NFPA) 78 (now superseded by NFPA 780) (2001).

Table 1 Categorization of lightning-induced LERs from 1990 to 2000.

Event Frequency

Reactor trip/scram 11Loss of power/voltage sag/diesel generator start 9

Ventilation isolation 2

Containment isolation 2Other 6

All events 30

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3. REVIEW OF APPLICABLE STANDARDS

A few key standards taken together cover the basics of lightning protection in

power generation stations. These standards cover external grounding grids and lightningprotection (IEEE Std. 665), conductor egress and internal grounds (IEEE Std. 1100, IEEE

Std. 1050, and IEEE Std. 666), and the proper selection and use of surge protectivedevices (SPDs) (IEEE Std. C62.23 and IEEE Std. C62.41). In addition to these standards,

several other standards address key concerns that are not completely covered by theaforementioned standards. A list of the 16 standards judged most applicable to lightningprotection for NPPs is given in Table 2.

IEEE Std. 665 was chosen as the primary standard for endorsement because of its

focus on grounding in generation stations. It draws from IEEE Stds. 80, 81, and 81.2 for

measurement and calculation of soil resistivity and grid currents. IEEE Std. 665 alsodraws from standards that focus on the grounding of specific types of systems; theseinclude IEEE Std. 142 for commercial power systems, IEEE Std. 1050 for I&C systems,

IEEE Std. C37.101 for generator grounds, and IEEE Std. C62.92 and IEEE 666 for

neutral grounding. Other standards highlight some issues, such as conductor egress andSPDs, more thoroughly than IEEE Std. 665 and are included as references.The six primary standards recommended for endorsement by this study are listed

in Table 3 with comments regarding their applicability. In addition, referenced materialfrom other standards is included in the recommendation.

Table 2 The 16 standards judged most applicable to lightning protection for NPPs.

Standard number Standard title

IEEE Std. 80-2000 IEEE Guide for Safety in AC Substation Grounding (ANSI)

IEEE Std. 81-1983 IEEE Guide for Measuring Earth Resistivity, GroundImpedance, and Earth Surface Potentials of a Ground

System (ANSI)IEEE Std. 81.2-1991 IEEE Guide for Measurement of Impedance and Safety

Characteristics of Large, Extended or Interconnected

Grounding Systems

IEEE Std. 142-1991 IEEE Recommended Practice for Grounding of Industrial

and Commercial Power Systems (IEEE Green Book)

IEEE Std. 367-1987 IEEE Recommended Practice for Determining the Electric

Power Station Ground Potential Rise and Induced Voltage

from a Power Fault (ANSI)

IEEE Std. 487-2000 IEEE Recommended Practice for the Protection of Wire-

Line Communication Facilities Serving Electric Power

Stations (ANSI)IEEE Std. 665-1995

(reaffirmed 2001)

IEEE Guide for Generating Station Grounding

IEEE Std. 666-1991 IEEE Design Guide for Electric Power Service Systems forGenerating Stations

IEEE Std. 1050-1996 IEEE Guide for Instrumentation and Control EquipmentGrounding in Generating Stations (ANSI)

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Table 2 (continued).

Standard number Standard title

IEEE Std. 1100-1999 IEEE Recommended Practice for Powering and GroundingSensitive Electronic Equipment (IEEE Emerald Book)

[ANSI]

IEEE Std. C37.101-1993

IEEE Guide for Generator Ground Protection (ANSI)

IEEE Std. C57.13.3-1983

IEEE Guide for the Grounding of Instrument TransformerSecondary Circuits and Cases (ANSI)

IEEE Std. C62.23-1995 IEEE Application Guide for Surge Protection of ElectricGenerating Plants (ANSI)

IEEE Std. C62.92-1987(reaffirmed 1993)

IEEE Guide for the Application of Neutral Grounding inElectrical Utility Systems, Part I-Introduction (ANSI)

IEEE Std. C62.41-1991

(R1995)

IEEE Recommended Practice on Surge Voltages in Low-

Voltage AC Power Circuits

NFPA 780-2001 Installation of Lightning Protection Systems

Table 3 Top standards related to lightning protection.

Standard Applicability

IEEE Std. 665-

1995 (R2001)

Recommended for endorsement as the primary guidance for

lightning protection in nuclear power plants. It focuses on the

direct effects of lightning strokes and draws heavily fromANSI/NFPA 780, Installation of Lightning Protection Systems.

ANSI/NFPA 780 is a widely accepted standard for lightning

protection of most types of structures, but it specifically excludes

power generation plants.IEEE Std. 666-

1991

Recommends neutral grounding practices for the grounding of

generating station auxiliaries, insulation levels for protectionagainst voltage surges from lightning transients, and surge

protection for transformers, switchgear, and motors.

IEEE Std. 1050-

1996

Covers the specific components necessary to prevent damage to

I&C equipment that can be caused by the secondary effects of

lightning.

IEEE Std. 1100-

1999

Covers more completely possible conflicts between the grounding

needs of I&C systems and those of other electronic systems(known as balance-of-plant equipment).

IEEE Std. C62.23-1995 (R2001)

Consolidates many electric utility power industry practices,accepted theories, existing standards/guides, definitions, and

technical references as they pertain to surge protection of electric

power generating plants.

IEEE Std. C62.41-

1991 (R1995)

Gives general guidance on the selection of surge-protection

devices that can reduce the effects of lightning-induced surges to

levels that electronic equipment can withstand.

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4. RESULTS

Experience shows that all nuclear power facilities should have a well-designed, -

installed, and -maintained lightning protection system (LPS), which traditionally refers toan external system consisting of air termination (lightning rods), down-conductors, and

an earth grounding system. Additionally, facilities containing vulnerable electronicequipment (such as I&C and other equipment that interconnects multiple signal and

communication paths) require an internal grounding system that addresses cable routingand bonding to the earth grid at key locations. The best-known source of informationabout LPS design guidelines in use today is ANSI/NFPA 780 (2001), which is the

foundation document for protecting facilities from direct lightning strikes. Almost all

lightning protection guidance standards reference ANSI/NFPA 780. However, while

ANSI/NFPA 780 gives good guidance and philosophies on lightning protection, it has adisclaimer concerning electric power generation facilities:

A-1-1.2 Electric generating facilities whose primary purpose is to

generate electric power are excluded from this standard with regard togeneration, transmission, and distribution of power. Most electricalutilities have standards covering the protection of their facilities and

equipment. Installations not directly related to those areas and structures

housing such installations can be protected against lightning by theprovisions of this standard.

Thus, while the concepts of the standard can be adopted, ANSI/NFPA 780 itselfcannot be endorsed as primary guidance for NPPs. It can, however, be used as a guide to

ensure all of the key elements of lightning protection are covered when other standardsare endorsed.

Key common sense guiding principles for protection against lightning taken from

the standards involve defense in depth and are almost always applicable:

1. If it is metal and is not intended to carry current, ground it.2. If it is metal and is intended to carry current, and it is outside a building, protect it

with taller grounded structures.3. If it is inside a building, surge-protect it.4. If it is a sensitive electronic circuit, build it to withstand whatever gets past the

barriers 13, including transients carried through low-level instrumentation wiring.

The seven themes listed in Table 4 represent a practical approach to meeting theguiding principles and determining whether or not an NPP has sufficient protection

against lightning.From a review of the applicable standards, we envision that IEEE Std.. 665-1995

(R2001) leads the other standards and gives guidance on protection of the whole facility.

IEEE Std. 666-1991 addresses neutral grounding and insulation. IEEE Std.. 1100-1999

addresses filtering and grounding of service lines and other conductors that egress theLPS boundary. IEEE Std. 1050-1996 applies to grounding and filtering I&C equipment

inside a facility. Finally, we use IEEE C62.23 and IEEE C62.41 to address

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Table 4 Key lightning protection themes.

Theme

1. Overall grounding plan2. Quality of LPS

3. Quality of filtering and grounding of conductors that egress LPS

4. Cable routing within the facility5. Correct selection and placement of SPDs throughout facility

6. Grounding of the I&C components

7. Protection of equipment from electromagnetic surges

surge-protection devices. While these standards address the major issues, further

guidance on secondary issues can be found in the other standards called out in Table 2.Data from the first LER examination period (19801991) indicate a probability of

3.2 104 per reactor year for a loss of fire protection with simultaneous fire due to a

lightning strike. The core damage probability (CDP) is estimated to be two orders of

magnitude less than that of the initiating event frequency, or 3.2 106

per reactor year.

Although this CDP value is small and may tempt analysts to conclude that lightningposes no safety concern, other factors should be considered. For example, electronicsystems are being designed with increased intelligence and are potentially more

vulnerable to electrical transients and EMI (NRC, 2000b; Ewing and Antonescu, 2001).

LER data are based on operating plants with older, analog electronic and

electromechanical systems. No data are available from plants with digitally-basedelectronic systems. Lightning transients have the potential of generating a cascade of

destructive events that generate spurious I&C signals while simultaneously damagingplant components. For example, lightning-induced electrical transients can erroneously

start subsystems and initiate fires. A particular event sequence that illustrates the point

occurred at a nuclear power station in June 1991 (NRC, 1991). Plant operating power was89 percent of full power prior to the event. The event was initiated by a lightning strike

(possibly multiple strikes) that disabled both off-site power sources, started a transformerfire, and disabled communication systems. The turbine and reactor automatically tripped

at the event onset. The cascade of failures delayed restoration of off-site power for about

12 hours.

5. CONCLUSIONS

Six IEEE standards are recommended for endorsement to address issues

associated with protecting NPPs and their equipment against lightning. In addition,referenced material from other standards is included in the recommendation.

Although an analysis of lightning-induced events at NPPs does not conclusivelyindicate imminent danger from lack of protection guidance, events at several facilities

suggest the need for more guidance for new facilities. NPPs built in the 21st century will

no doubt employ state-of-the-art electronics having a multiplicity of signal andcommunication interconnections, with their attendant sensitivity to electromagnetic

interference. It would, therefore, be prudent to develop an appropriate technical basis forlightning protection practices.

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ACKNOWLEDGEMENTS

The authors wish to thank Christina Antonescu, JCN W6851 Project Manager, of

the NRC RES for her help in initiating, planning, and implementing this research effort.Research was sponsored by the U.S. Nuclear Regulatory Commission and performed by

Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Departmentof Energy under contract DE-AC05-00OR22725.

REFERENCES

ANSI/NFPA (American National Standards Institute/National Fire Protection

Association) 2001. Standard for the Installation of Lightning Protection Systems,

ANSI/NFPA 780-2001.

Ewing, P. D. and C. Antonescu 2001. EMI/RFI and Power Surge Withstand Guidancefor the U.S. Nuclear Regulatory Commission, CNRA/CSNI Workshop, Hluboka

nad Vltavou, Czech Republic, Committee on Nuclear Regulatory Activities and the

Committee on the Safety of Nuclear Installations. Programme Committee andSession Chairmen will publish the summary report, Hluboka nad Vltavou, Cz.

2001.

LPI (Lightning Protection Institute) 1999. Lightning and Lightning Protection Systems,

http://www.lightning.org/protect.htm.

NRC (U.S. Nuclear Regulatory Commission) 1991. Loss of Offsite Power Event, NRC

Special Inspection Report 50-29/91-09, July.

NRC (U.S. Nuclear Regulatory Commission) 2000a. Guidelines for Evaluating

Electromagnetic and Radio-Frequency Interference in Safety-RelatedInstrumentation and Control Systems, Regulatory Guide 1.180, January 31.

NRC (U.S. Nuclear Regulatory Commission) 2000b. Recommended ElectromagneticOperating Envelopes for Safety-Related I&C Systems in Nuclear Power Plants,

NUREG/CR-6431, January.

Nuclear Reactor Shuts Down after Lightning Strike, Ananova,

http://www.ananova.com/news/story/sm_39104.html, August 18, 2000.

Pielage, H. W. 1981. Lightning Problems and Protection at Nuclear Power Plants,

NSAC-41, Nuclear Safety Analysis Center, Electric Power Research Institute,December.

Rourk, Chris 1992. Report on the Sources and Effects of Electrical Transients on theElectrical Systems of Commercial Nuclear Power Plants, Internal Engineering

Report, Engineering Issues Branch, Division of Safety Issues Resolution, Office ofNuclear Regulatory Research, U.S. Nuclear Regulatory Commission, September.

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Rourk, Chris 1994. A Review of Lightning-Related Operating Events at Nuclear Power

Plants,IEEE Transactions on Energy Conversion, 9(3), 636641 (September).

Schill, Karin 1996. SRS Frequent Lightning Target, Augusta Chronicle Online,

http://www.augustachronicle.com/headlines/091796/srs_lightning.html,September 16.

Tenth Annual Report on the Safety of Diablo Canyon Nuclear Power Plant Operations,July 1, 1999June 30, 2000.

Uman, M. A, 1984. Lightning, ISBN 0-486-64575-4, Dover. General Publishing

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Edoc - A Technical Basis for Guidance on Lightning Protection for NPP